Back-streaming control in a diffusion pump



Nov. 21, 1967 T. E. BOWMAN ET L 3,353,742

I BACK-STREAMING CONTROL IN A DIFFUSION PUMP Filed Oct. 14, 1965 g D Q Dc 1 D G\ 1/2 5 Q 22 i Q\ 2 6- j gg 2a I (30 I3 n POWER x N SUPPLYINVENTORSY I THOMAS E. BOWMAN BY PATRICK B. KENNEDY ATTORNEY UnitedStates Patent Energy Commission Filed Oct. 14, 1965, Ser. No. 496,202 1Claim. (Cl. 230-401) This invention relates to a diffusion type vacuumpump in which a selected granular material is disposed in the boilerwell region between the jet stack and the pump housing to a depthgreater than the pumping fluid. The material impedes eruptive boiling ofthe pumping fluid and thus greatly reduces this principal cause ofback-streaming of the pumping vapor into the vacuum system.

This invention relates to high vacuum diffusion pumps and moreparticularly to decreasing the amount of backstreaming experienced inthe operation of diffusion pumps whereby higher vacuums may be obtainedtherewith. The invention described herein was made in the course of, orunder, Contract W-7405-eng-48 with the United States Atomic EnergyCommission.

Diffusion pumps, which utilize jets of vaporized oil or mercury towithdraw gas from a chamber to be evacuated, are very extensively usedto provide high vacuums and many attempts have been made to improve pumpperformance from the standpoint of reducing back-streaming, i.e., thedrifting of pumping fluid vapor into the vacuum chamber being pumped.These attempts at improvement have included the use of specially shapedjet nozzles, the use of special heating devices to superheat the vaporbefore or during its passage through the nozzles and also the use of arefrigerated cap over the nozzle support structure. Such changes haveimproved operation of the pumps in some respects but have not eliminatedthe back-streaming which has continued to present a problem in highvacuum pumping. The rate of back-streaming in fact is believed to be thefactor which limits the degree of vacuum which can be obtained with thistype of pump.

In most diffusion pumps, the pumping fluid boiler is situated beneaththe vapor jets and a portion of the boiling liquid, i.e. the portionbetween the shell of the pump and the jet forming tubulation whichextends upwardly therein, is directly exposed to the vacuum chamberexcept for the presence of the jets. It has now been found that aprincipal cause of back-streaming is eruptive boiling in the boilerwhich impels pumping fluid upwardly towards the jets and ultimately intothe region which is being evacuated.

In the present invention, by a fluid pervious filling of a material suchas activated alumina granules in the bottom of the pump to a levelhigher than that of the boiling pumping fluid, a large improvement ismade in reducing the amount of back-streaming. For example a reductionto one part from a previous amount of 1,000 to 10,000 parts ofback-streaming pump oil has been achieved in a test where the granulefilling was largely confined to the exposed portion of the boiler, i.e.the area of the boiler between the jet forming tubulation and the outershell.

One reason this result is obtained is that the granules mechanicallyimpede the violent upward movement of fluid and break up the largebubbles which otherwise result from eruptive boiling of the liquid inthe base of the pump. Also it is believe-d that nucleate boiling in thepresence of activated alumina granules causes vapor to be given off fromthe liquid in small amounts and in small bubbles thus avoiding the largeconcentrations of superheat which cause severe eruptive boiling wherethe granules are not present.

Although activated alumina granules provide excellent results, othermaterials in granulated form are also suitable. Tests using glass beadsof a size about one fourth inch in diameter produced a large reductionin backstreaming but failed to reach the amount of reduction achievedwith the granules of alumina by about one order of magnitude. The reasonfor less reduction of backstreaming in the instance of the use of glassbeads is believed to be that these beads merely produce the mechanicalimpediment to the production of large bubbles and severe eruptiveboiling and do not produce the nucleate boiling obtained with theactivated alumina granules.

The invention eliminates the requirement for a cold trap between thediffusion pump and the vessel being evacuated in many applications whereit has heretofore been required and this in turn greatly increases thepumping speed in these applications. A further important advantage ofthe invention is that it may be readily applied to many existing pumpsand is not limited only to specially constructed new pumps.

Therefore it is an object of the present invention to increase theefficiency of operation of a diffusion pump.

A further object of this invention is to reduce backstreaming in adiffusion pump.

Another object of the present invention is to increase the degreeof'vacuum that can be obtained with diffusion pumps.

A still further object of this invention is to reduce the requirementfor the use of cold traps in high vacuum pumping.

The invention together with further objects and advantages thereof willbe best understood by reference to the following specification inconjunction with the accompanying drawing which is a broken-out view ofa typical diffusion pump showing the invention embodied therein.

Referring now to the drawing an upright tubular shell 11 having a flange12 at the top and a flat base 13 forms the outer shell of the diffusionpump. Flange 12 connects to a flange 14 associated with entrance tube 16leading to the vacuum chamber 17 which is to be evacuated. Water tubing18 coiled around the outside of tubular shell 11 cools the diffusionpump to condense the pumping fluid vapor which impacts against theinside surface of the shell. A small connecting tube 19 sealed into theside of the tube 11 near but somewhat up from the base 13 provides forconnection to a forevacuum pump by means of flange 21.

A vapor jet forming tubulation 22, smaller than the outer tubular shell11, is located inside the shell 11 in coaxial relationship thereto andis supported on the base 13. A series of notches 23 are provided in thelower edge of tubulation 22 to permit pumping fluid 25 to pass betweenthe outside and inside of the tubulation at the bottom thereof.Tubulation 22 has annular nozzles 24 formed therein in the usual mannerfor diffusion pumps, three such nozzles being employed in the presentinstance. The nozzles 24 are vertically spaced and are coaxial with thetubulation 22 to provide the customary annular vapor jets directedoutwardly and downwardly towards the inside wall of shell 11. A pumpingoil 25 is provided in the bottom of the shell 11 and tubulation 22 to alevel 26 which is suflicient to more than cover apertures 23. Anelectric heater 27, coupled to a power supply 30, is disposed againstthe base 13 of shell 11 and heats the pumping oil 25. Vapor from theheated pumping fluid rises into the tubulation 22 and is ejected throughthe nozzles 24 to form the above-described jets.

Back-streaming arises primarily from the surface areas 26 of the pumpingoil 25 outside the tubulation 22, between the tubulation and the shell11. Eruptive boiling in this area may move pumping fluid particlesdirectly into the pumped region thereby limiting the vacuum obtained.The vacuum is further contaminated by the release of gas molecules whichhave been absorbed in the pumping fluid. To overcome this condition afilling of small granules 28 is placed in the bottom regions of thepump, at least in the space between the tubulation 24 and the outershell 11, to a depth well above the pumping fluid surface'level 2.6.

The primary purpose of the granules 2.8 is to physically impede .theviolent upward movement of pumping fluid which is associated witheruptive boiling and thus the granules may be formed of any materialhaving suitable mechanical strength, heat resistance, and which does notreact with the pumping fluid. Glass beads, for example, have been foundto be satisfactory. However, particularly goodresults have been obtainedwhere the granules 28 are formed of activated alumina. As hereinbeforediscussed, it is believed that nucleate boiling in the presence ofactivated alumina causes vapor to be given ofi in smaller units andreduces the concentrations of superheat which are otherwise present. Theactivated alumina granules 28 may typically be one-eighth inch in sizeor somewhat larger. Smaller sizes have been found to be lesssatisfactory.

The use of the activated alumina particles 28 has been made to reduceback-streaming by a large amount, approximately of the orderofone-thousandth to one-tenthousandth of the amount encountered beforethe use of the activated alumina.

Many variations are possible within the spirit and scope of theinvention and it is not intended to limit the invention except asdefined in the following claim.

What is claimed is:

A difiusion pump having an outer shell with a pumping fluid disposedtherein together with means for heating said fluid and having an innertubulation forming at least one vapor jet directed towards said shelland away from the region to be evacuated and wherein said pumping fluidin the annular space between said shell and said tubulation is exposedto said region to be evacuated comprising; a filling of loose granularmaterial disposed in the lower portion of said pump at'least insaidannular space and exposed directly to said region to be evacuatedbetween said tubulation and said outer shell and extending to a depthgreater than the depth of said pumping fluid therein wherebyback-streaming is reduced.

References Cited UNITED STATES PATENTS 655,274 8/1900 Ramsden 122-3671,874,537 8/1932 Jaycox 230-101 2,437,849 3/1948 COX et a1. 230-41012,438,387 3/1948 Colaiaco 230--101 2,886,235 5/1959 Denton 230 1013,251,537 5/ 1966 Bancroft et a1. 230-101 3,258,196 6/1966 Knox et al.230101 DONLEY J. STOCKING, Primary Examiner.

ROBERT M. WALKER, Examiner.

W. I. KRAUSS, Assistant Examiner.

